How to make an arch in a model w/o Bezier Curves and w/o adding separate objects?
I've kept your topology but no need for so many edge loops. Anyway, you could use the Bridge tool:


First, tilt a bit the top faces of your left and right columns.
Press W > LoopTools > Bridge, or W > Bridge Edge Loops.
Play with the parameters on the bottom of the Tools panel.
Fill the hole the way you want to finish the work. Here are 2 ways to do it, maybe there's a better one though.